Wireless LANs are usually based on a medium access control (MAC) using a listen-before-talk scheme like carrier sense multiple access with collision avoidance (CSMA/CA) as described by the IEEE 802.11 standard. Such a scheme allows a station to start a transmission when there is no transmission active that is being received above a certain threshold level.
The IEEE 802.11 standard for wireless LANs is a standard for systems that operate in the 2,400-2,483.5 MHz industrial, scientific and medical (ISM) band. The ISM band is available worldwide and allows unlicensed operation for spread spectrum systems. IEEE 802.11 focuses on the MAC and physical layer (PHY) protocols for access point based networks and ad-hoc networks. IEEE 802.11 supports direct sequence spread spectrum (DSSS) with differential encoded BPSK and QPSK, frequency hopping spread spectrum (FHSS) with Gaussian FSK (GFSK), and infrared with pulse position modulation (PPM).
The basic medium access behavior allows interoperability between compatible physical layer protocols through the use of both CSMA/CA and a random backoff time following a busy medium condition. In addition, all directed traffic uses immediate positive acknowledgment (ACK frame), where retransmission is scheduled by the sender if no ACK is received. The CSMA/CA protocol is designed to reduce the collision probability between multiple stations accessing the medium at the point where they would most likely occur. The highest probability of a collision occurs at the moment in time that is just after the medium becomes free following a busy medium, because multiple stations are waiting for the medium to become available. Therefore, a random backoff arrangement is used to resolve medium contention conflicts. Basic CSMA/CA medium access control scheme behavior is shown in FIG. 1. IFS stands for Inter Frame Spacing.
However, the MAC cannot always prevent the occurrence of co-channel transmissions that overlap in time due to position dependent receive level variations or limited margins with respect to the carrier sense/defer threshold.